Microwave applicator

ABSTRACT

An applicator for microwave energy employing a heating chamber with inlet and outlet openings and an attenuation channel communicating with one of these openings. A conveyor is arranged to carry material to be treated in and out of the openings and through the channel. A feature of the applicator is the provision of an attenuation section in the channel which has spaced plates forming passages containing microwave absorbing material. Another feature is the provision of an intermediate attenuation section wherein a liquid which is lossy to microwaves is circulated into access with microwave energy within the intermediate section.

United States Patent 1191 Smith Dec. 31, 1974 MICROWAVE APPLICATORPrimary Examiner-Bruce A. Reynolds [75] Inventor. Franklin J. Smith,D1ablo, Cal1f. Attorney Agent, or Firm carl C. Batz [73] Assignee:Microdry Corporation, San Ramon,

Canada [57 ABSTRACT [22] Filed: 219-1972 An applicator for microwaveenergy employing a heat- 2 APPL 24 452 ing chamber with inlet and outletopenings and an attenuation channel communicating with one of theseopenings, A conveyor is arranged to carry material to [52] US Cl219/1055 9 E be treated in and out of the openings and through the [51]Int. Cl. HOSb 9/06 Channel A feature of the apphcator is the provisionof [58] Fleld of Search 219/1055 an attenuation Section in the Channelwhich has spaced plates forming passages containing microwave [56]References C'ted absorbing material. Another feature is the provision ofUNITED STATES PATENTS an intermediate attenuation section wherein aliquid 3,050,606 8/1962 Tibbs 219/1055 which is lossy t0 microwaves iscirculated into access 3,427,171 2/1969 Jeppson.. 219/1055 X withmicrowave energy within the intermediate sec- 3,624,335 11/1971 Dench219/1055 tion. 3,665,141 5/1972 Schiffmann 219/1055 3,754,111 8/1973Jurgensen 219/1055 15 Claims, 4 Drawmg Figures MICROWAVE SOURCE A B F TMW .42 4O 1o C-l C.la l4 2 3 02 J "MICROWAVE SOURCE F IGJ FIG.3

BACKGROUND Microwave ovens wherein microwave energy is applied tofoodstuffs and other materials to be heated are now well-known. Some ofthe ovens are of the batch type where materials are put in the overn,the door closed, and the microwave energy turned on. Other ovens now incommercial use are designed for continuous operation and in which thematerial to be treated is passed on a conveyor through an inlet openingthrough a heating chamber or cavity and then out through an outletopening. Such a continuous applicator is shown in the US. Pat. No.3,365,562. to .Ieppson.

Because it is necessary in the microwave applicators of the continuoustype to have openings through which material can be passed into and outof the-heating cavity there has been a problem of keeping the microwaveenergy from leaking out through these openings. This leakage representsa loss of energy but the more important consideration is that the energywhich leaks out in this way passes into the atmosphereand presents ahazard to persons who may be in the vicinity. As the use of continuousmicrowave applicators has become greater more notice is taking of theradiation hazard and there is needed in this art apparatus which willavoid'the emission of even any smallamounts of microwave radiation intothe atmosphere.

Apparatus is described in U.S. Pat. No. 3,365,562 which includes an endsection into whichwater or other lossy liquid is circulated sothatenergywhich tends to leak out of the treatmentchamber comes into access withthe water and the energy so absorbed in the water is carried away.Although this attenuation substantially reduces the leakage thereremains a small but still objectionable amount of energy which is passedinto the atmosphere, and what is desired is an improved structure whichwill stop the passing of even small amounts of energy into theatmosphere while at the same time avoiding any objectionable reflectionsback to the system. What is needed is some simple structure easilyfabricated which will accomplish the desired purpose.

SUMMARY OF THEINVENTION I have discovered that by the inclusionin theapplicator of an attenuation section having a special lining andinternal passages the purposes above described can be THE DRAWINGS Oneembodimentof myinvention is shown in the accompanying drawingsin which:

FIG. 1 is a perspective view of my improved applicator apparatus;

FIG. 2 is an enlarged perspective view of the improved attenuationsection of the applicator with one corner broken away to shown theinterior structure;

FIG. 3 is an enlarged broken sectional view taken along the plane oflines 3-3 of FIG. 2; and

FIG. 4 is an enlarged broken sectional view taken along the plane ofline 4-4 of FIG. 2.

DETAILED DESCRIPTION As illustrated, the improved applicator includesthe central heating section A, an end attenuating channel Bhaving anintermediate attenuation section C-1 and a second attenuation sectionC-2.

The heating section A is formed by a metal casing 10 which isrectangular in cross section having spaced broad walls 11 at the top andbottom and sidewalls 12 at the front and rear. A source of microwaveenergy 13 communicates with the interior cavity 14 and microwave energypermeates this cavity. The walls 11 and 12 of the cavity are of metaland are good electrical conductors which means they reflect themicrowave energy back and forth across the cavity and so contain theenergy-within the cavity until it is absorbed by the product beingtreated or leaks out through the openings which necessarily areprovided.

A conveyor 15 having a belt 16 made of a material such as Teflon, whichisa plastic having the characteristic of being transparent to microwaveenergy. This.

belt may be made of any flexible material which has thisfcharacteristic.The belt is shown asrunning be; tween the rollers 17 and 17a, andfor-purpose ofexplanationis taken as moving in the direction shown byarrows 18.

Pieces of food or other material to be treated, 19, may be placed on theconveyor belt and through opera tion of the conveyor pass through theinlet opening 20 into the heating cavity and then out of the heatingcavity through opening 21. In FIG. 1 a door 22 is shown in open positionexposing the material 19 being conveyed longitudinally of the heatingcavity. The door is for purposes of illustration only and is unnecessaryfor the proper operation of the applicator. The length of the heatingcavity should be sufficient to allow for the desired heat treatment ofthe material 19 as may be required, taking into consideration the speedof the conveyor, etc.

The attenuation channel B communicates with the interior of the heatingcavity at opening 21 and has its section C-l adjacent this cavity. Onthe interior of section C1 is a structure 23 of rectangular crosssection made suitably of a plastic material which is transparent tomicrowave energy and about this is a tube 24 also made of a plastic orother material transparent to microwave energy and wound about thisstructure 23. Water or other lossy material may be circulated throughthe tube 24.

The structure 23 communicates on its interior with the opening 21 andthe conveyor belt 16, with the material l9 thereon, passeslongitudinally through structure 23;

Microwave energy which leaks out of opening 21 from the interior ofcavity 14 may pass through the walls of structure 23 through the wallsof tube 24 and be absorbed in the water or other lossy liquid circulatedthrough this tube. The outer walls of section C-l, being metallic,reflect any energy which may reach them back into the lossy liquid. Thusthere is substantial attenuation of the energy which leaks throughopening 21.

The attenuation channel B has its second attenuation section C2 adjacentto section Cl with its interior communicating through outlet opening 25of section C-1, and the opening 21, with the heating cavity. Thissection has a metal outer casing which is rectangular in cross sectionhaving top and bottom broad walls 26 and 27 and sidewalls 28 and 29 (seeparticularly FIGS. 2 to 4). These walls are metallic and, therefore,reflect microwave energy. Lining the top wall 26 is a sheet 30 which maybe a composition of materials including powdered iron and inertmaterials bound together with a binding material such as an epoxy resin.Such a material is manufactured by the Microdry Corporation of SanRamon, Calif. and sold under the' trademark CRYOCOTE. This compositematerial is a good absorber of microwave energy and has a dialecticconstant of between 1 and 10. The sheet 30 of this material is securedto the top wall 26 by an adhesive such as an epoxy adhesive. Likewisesheet 31, of similar material is secured to the bottom broad wall 27 andsheets 32 and 33 are secured to the left and right sidewallsrespectively. Each of these sheets of absorbing material may be of theorder of about A inch thick, for example. Skid strips 31a extend overthe lining layer 31 and the conveyor belt 16 may slide over these skids.

Inside the section C2 are the spaced metal plates 35 which extendhorizontally between the sides of the cas ing transversely of the endchannel and transverse to g the general direction of the flow of anyenergy tending to pass through the channel. These plates are supportedat their ends by the support members 36 which are secured to the casing.

As seen more clearly in FIG. 4 the spaced plates 35 provide between themthe passages 37 which lead from the interior of section C2 and extendupwardly to the top wall. The ends of these passages are closed by thelayer of lossy material 30 carried by the top wall. I prefer that theplates 35 be mounted in parallel arrangement with even and uniformspacing between them so that the vertical passages 37 will berectangular in cross section from their open bottom ends to their topends.

Secured on one side of each of plates 35 and within each of passages 37is a layer or strip of lossy material 38. This strip 38 extends alongsubstantially the whole side of plate 35 and its top edge meets the toplayer 30. It may have a thickness such as about a /1 inch, for example.The lower edges 35a of the plates are turned rearwardly so as to comeunder the edges of strips 38 and serve to hold these strips. Further, itis an improved feature that the end portions of plates 35 be turnedforwardly as indicated at 35b of FIG. 3 so that the inturned edgeportions form at least a part of the sides of the passageways 37, andthe edges of these portions 35b abut against the strips 38 attached toplates 35 so as to determine the spacing between the plates and keep theplates in their uniform, parallel, evenly spaced relation. Thisstructure also, helps in manufacture since the plates may be placedsimply one against the other, without the need for measurement.

The height of plates 35 and strips 38, and consequently the depths ofthe passages 37, should be determined to a substantial extent by thefrequency used in exciting the heating chamber. Frequencies in commonuse for industrial heating purposes may be either 2450 HMz or 915 HMz,these frequencies being approved for such purposes by governmentagencies; but my invention may, of course, be used with otherfrequencies. These passages are dead-end passages, having their upperends blocked by lining 30 and top plate 26. l prefer that the width ofthe metallic plates 35 and therefor the depth of the passages 37, be ofthe order of A the wave length of the selected frequency. This depth maybe within the range of /8 to wave length, preferably not greater than /2wave length of the predetermined frequency.

' To demonstrate the preferred size of my improved attenuation section Itake, for example, an excitation frequency of 2450 HMz. For thisfrequency I provide a section about 8 inches long with a breadth ofabout 10 inches and a height of about 4 /2 inches. This accommodates anend opening of about 3 inches in height with space in the upper part ofthe casing for the special internal structures. The sheets which providethe internal linings along the walls may be about one-fourth inch thick.The plates 35 may be about one-half inch apart and extend in heightabout 1 inch (the wave length for 2450 I-IMz is about four inches). Thelossy strips which are secured to the plates 35 may also be aboutone-quarter inch in thickness.

' OPERATION In operation, the microwave energy which passes through thefirst attenuation section C-l enters the interior of section C2 where itmeets the lossy dry material of sheets 31, 32 and 33 lining the interiorof the metal casing, and also passes into the mouths of passages 37between plates 35 and proceeds by reflection between the plates 35 untilit is absorbed by the material 38 or by the material 30 at the end ofthe passages. Substantially all of the microwave energy which enterssection C2 is absorbed therein, with practically none being permitted tobe discharged to the atmosphere. We have found that the attenuationapplicable to the reduced amounts of the energy passed into section C2is very efficient, reducing by a factor of more than 5 to l the energywhich is permitted to be passed on.

We find that while the provision of the water load in section Cl isefficient to remove most of the microwave energy tending to leak fromthe heating chamber it is not practical to attempt to remove enough ofthe energy by this method to satisfy all requirements. Further, it isnot practicable to have in addition to the water load attenuation anydevice such as a reject filter. A reject filter is necessarily limitedin size to a wave guide propagating energy in a single mode at thefrequency which has been selected. What is desired is a device such as Ihave described which will handle multi-mode transmission of energy, andwhich can be constructed in a practical size to accommodate the approvedfrequency ranges.

In the foregoing description I have described an attenuation channelstructure B which is in communication with the outlet opening from theheating cavity. It may be observed that if the conveyor belt 16 were runin the opposite direction the opening 21 would, in such case become theinlet opening and the attenuator section C-l and C2 would be guardingagainst leakage of energy issuing from the inlet opening to the heatingcavity.

I prefer to provide anattenuation section Cla similar to section C-l butguarding against energy passing through the opening and anotherattenuation section similar .to section C-2 which can'be adjacentsection C-la to attenuate energy which may pass through section C-la.

Conveniently, water may be circulated into the coil 24 through pipe 40and then into the coil 24a of section C-la by means of pipe 41 anddischarged through pipe 42.

Further, and particularly if the amount of energy leaking throughopening 21 is not great, the attenuator section C-2 may be usedexclusively for absorbing the energy leaking through the opening of theheating cavity. This may be done by not circulating the lossy liquidthrough section C-l or by omitting this section altogether so that allenergy passing through the outlet opening in the heating cavity entersthe heating section C-2 and is attenuated therein.

I believe that the efficiency of my improved attenuating section C-2 maybe due in part to the fact that the microwave energy absorbing materialcontained therein has a dialectic constant of from 1 to 10, usuallybetween 2 to 5, which contrasts with the dialectic constant of waterwhich is 80 or more, so that at the interface between air and water thedialectic ratio is about 80 to 1 whereas at the interface between airand absorbing material the dialectic ratio is usually not more than 5to 1. This condition is believed to permit less reflection of energywhen directed against our absorbing material (and consequently moreefficient absorption) than would be the case where the energy movesagainst water. Further, the passages within my section C-2 between theplates 35 must receive the energy which moves toward the top broad wall,and this energy is not reflected back but it is absorbed therein. Myimproved attenuation section C-2 may, therefore, be used as the solemeans of attenuation whenever the energy to be dissipated is not sogreat as to develop a large amount of heat. When the energy to bedissipated is so large as to require special radiation and carrying awayof heat I prefer to use the 0-1 section in front of the C-1 attenuatorsection.

in the foregoing detailed description I have set forth preferredembodiments of my invention and have given specific examples ofstructure and modes of practicing the invention, but it is understoodthat many changes and variations may be made, all within the spirit ofthe invention and scope of the appended claims.

What I claim is:

1. Microwave Applicator Apparatus comprising a casing providing aheating cavity and having openings through which material to be treatedmay be passed into and out of said cavity, means for exciting saidcavity with microwave energy, an attenuating channel having its one endin communication with one of said openings and having at its other endan opening through which said material may be passed, said channelhaving conducting outer walls and having at one section thereof betweensaid ends a lining adjacent to said outer walls formed of a sheetmaterial which is lossy to microwave energy, a flat side of said sheetmaterial being in contact with said walls, said section containing aplurality of passageways leading from the interior thereof to one ofsaid outer walls, said passageways being formed by spaced plates ofconducting material extending across said section.

2. Microwave Applicator Apparatus comprising a casing providing aheating cavity and having openings through which material to be treatedmay be passed into and out of said cavity, means for exciting saidcavity with microwave energy, and an attenuating channel having its oneend in communication with one of said openings and having at its otherend an opening through which said material can be passed, said channelhaving conducting outer walls and containing at one section thereof aplurality of passageways leading from the interior of the channel to oneof the outer walls, said passageways being formed by spaced plates ofconducting material extending across said section.

3. Apparatus as set forth in claim 2 wherein each of said passagewayscontains a material which is lossy to microwave energy.

4. Apparatus as set forth in claim 2 wherein said passageways haveclosed ends and have a layer of said material across their ends andalong at least one sidewall thereof.

5. Apparatus as set forth in claim 2 wherein said section is rectangularin cross section with broad top and bottom walls and wherein saidpassageways are vertical and extend upwardly from the interior of saidsection to their ends at said top broad wall.

6. Apparatus as set forth in claim 2 wherein said means is for excitingsaid cavity at a predetermined frequency and wherein said passagewayshave a depth of from one-eighth to three-eighth of the wave length ofsaid predetermined frequency.

7. Apparatus as set forth in claim 6 wherein said passageways have adepth of about one-fourth wave length of said predetermined frequency.

8. Apparatus as set forth in claim 2 wherein said plates have a liningon one side thereof formed of a lossy material and wherein said platesare uniformly spaced and in planes parallel with each other whereby saidpassages are of uniform width throughout their depth.

9. Apparatus as set forth in claim 2 wherein said channel has anintermediate section between said heating cavity and said one section,means for circulating a lossy liquid through said intermediate sectionwhereby microwave energy passing from said cavity into said one sectionmay be at least partially absorbed in said lossy liquid.

10. Apparatus as set forth in claim 9 wherein said plates are lined onone side with a layer of lossy material.

II. In a microwave applicator an attenuator section having an openpassage from one end to the other through which materials to be treatedby the applicator may be passed, an outer metal casing, a plurality of vspaced metal plates. in said casing providing passages leading from theinterior to one wall of said casing, said plates having a lining on oneside thereof of a material lossy to microwave energy, whereby microwaveenergy entering said passages is absorbed in said lining.

12. Apparatus as set forth in claim 11 wherein said plates have theirend portions turned forwardly with the end edges abutting the adjacentplates whereby the spacing between said plates is determined by thelength of said forwardly turned end portions.

13. Apparatus as set forth in claim 11 wherein said frequency is about2450 HMZ and the depth of said passages is about 1 inch.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 353022 1 Dated December 31, 1974 Inventor(s) Franklin J. Smith It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Title page, line [73] "Canada" should read --California--.

Signed and sealed this 4th day of March 1975.

(SEAL) v Attest a C. MARSHALL DANN RUTH C. MASON Commissioner of PatentsAttesti'ng Officer and Trademarks :ORM PC4050 usco'MM-Dc 60376-P59 IU.S. GOVERNMENT PRINTING OFFICE: l9" 0-366-334 UNITED STATES PATENTOFFICE CERTIFICATE OF CORRECTION Patent No. 3 353 022 Dated December 31,1974 Inventor(s) Franklin J. Smith It is certified that error appears inthe above-identified patent and that said Letters Patent are herebycorrected as shown below:

Title page, line [73] '"Canada" should read --California--.

Signed and sealed this 4th day of March 1975 (SEAL) I Attest:

. C MARSHALL DANN v RUTH C. MASON Commissioner of Patents AttestingOfficer and Trademarks uscouwoc 60316-P69 ".5, GOVERNEMT PRINTINGol'lilc! I l". 0-366-334

1. Microwave Applicator Apparatus comprising a casing providing aheating cavity and having openings through which material to be treatedmay be passed into and out of said cavity, means for exciting saidcavity with microwave energy, an attenuating channel having its one endin communication with one of said openings and having at its other endan opening through which said material may be passed, said channelhaving conducting outer walls and having at one section thereof betweensaid ends a lining adjacent to said outer walls formed of a sheetmaterial which is lossy to microwave energy, a flat side of said sheetmaterial being in contact with said walls, said section containing aplurality of passageways leading from the interior thereof to one ofsaid outer walls, said passageways being formed by spaced plates ofconducting material extending across said section.
 2. MicrowaveApplicator Apparatus comprising a casing providing a heating cavity andhaving openings through which material to be treated may be passed intoand out of said cavity, means for exciting said cavity with microwaveenergy, and an attenuating channel having its one end in communicationwith one of said openings and having at its other end an opening throughwhich said material can be passed, said channel having conducting outerwalls and containing at one section thereof a plurality of passagewaysleading from the interior of the channel to one of the outer walls, saidpassageways being formed by spaced plates of conducting materialextending across said section.
 3. Apparatus as set forth in claim 2wherein each of said passageways contains a material which is lossy tomicrowave energy.
 4. Apparatus as set forth in claim 2 wherein saidpassageways have closed ends and have a layer of said material acrosstheir ends and along at least one sidewall thereof.
 5. Apparatus as setforth in claim 2 wherein said section is rectangular in cross sectionwith broad top and bottom walls and wherein said passageways arevertical and extend upwardly from the interior of said section to theirends at said top broad wall.
 6. Apparatus as set forth in claim 2wherein said means is for exciting said cavity at a predeterminedfrequency and wherein said passageways have a depth of from one-eighthto three-eighth of the wave length of said predetermined frequency. 7.Apparatus as set forth in claim 6 wherein said passageways have a depthof about one-fourth wave length of said predetermined frequency. 8.Apparatus as set forth in claim 2 wherein said plates have a lining onone side thereof formed of a lossy material and wherein said plates areuniformly spaced and in planes parallel with each other whereby saidpassages are of uniform width throughout their depth.
 9. Apparatus asset forth in claim 2 wherein said channel has an intermediate sectionbetween said heating cavity and said one section, means for circulatinga lossy liquid through said intermediate section whereby microwaveenergy passing from said cavity into said one section may be at leastpartially absorbed in said lossy liquid.
 10. Apparatus as set forth inclaim 9 wherein said plates are lined on one side with a layer of lossymaterial.
 11. In a microwave applicator an attenuator section having anopen passage from one end to the other through which materials to betreated by the applicator may be passed, an outer metal casing, aplurality of spaced metal plates in said casing providing passagesleading from the interior to one wall of said casing, said plates havinga lining on one side thereof of a material lossy to microwave energy,whereby microwave energy entering said passages is absorbed in saidlining.
 12. Apparatus as set forth in claim 11 wherein said plates havetheir end portions turned forwardly with the end edges abutting theadjacent plates whereby the spacing between said plates is determined bythe length of said forwardly turned end portions.
 13. Apparatus as setforth in claim 11 wherein said frequency is about 2450 HMz and the depthof said passages is about 1 inch.
 14. Apparatus as set forth in claim 11including a sheet of said lossy material closing said passageways atsaid one wall.
 15. Apparatus as set forth in claim 11 wherein saidmicrowave energy has a predetermined frequency and the depth of saidpassages is about one-eighth to three-eighths of the wave length of saidpredetermined frequency.